Reduction-oxidation (redox) flow batteries store electrical energy in a chemical form and subsequently dispense the stored energy in an electrical form via a spontaneous reverse redox reaction. A redox flow battery is an electrochemical storage device in which an electrolyte containing one or more dissolved electro-active species flows through a reactor cell where chemical energy is converted to electrical energy. Conversely, the discharged electrolyte can be flowed through a reactor cell such that electrical energy is converted to chemical energy. The electrolytes used in flow batteries are generally composed of ionized metal salts that are stored in large external tanks and are pumped through each side of the cell according to the charge/discharge current applied. Externally stored electrolytes can be flowed through the battery system by pumping, gravity feed, or by any other method of moving fluid through the system. The reaction in a flow battery is reversible, and the electrolyte can be recharged without replacing the electroactive material. The energy capacity of a redox flow battery, therefore, is related to the total electrolyte volume, e.g., the size of the storage tank. The discharge time of a redox flow battery at full power also depends on electrolyte volume and often varies from several minutes to many days.
The minimal unit that performs the electrochemical energy conversion is generally called a “cell,” whether in the case of flow batteries, fuel cells, or secondary batteries. A device that integrates many such cells, coupled electrically in series or parallel, to get higher current or voltage or both, is generally called a “battery.” As used herein, the term “battery” may refer to a single electrochemical cell or a plurality of electrically coupled cells. Like traditional batteries, cells may be “stacked” together in a flow battery system to achieve the desired power output. Thus, the terms “cell” and “battery” can be used interchangeably herein.
Since the electrolyte is stored externally, the amount of energy that can be stored by a flow battery is largely determined by the solubility of the chemicals and the size of the tanks. The size of the tanks and storage capacity can be easily scaled. A true flow battery has all chemical species flowing through the battery and stored in external tanks and thus the energy and volume capacities can be sized independently. The vanadium redox flow battery is an example of a true flow battery and has received the most attention in recent years. In a hybrid flow battery, at least one of the chemical states resides within the stack such as by plating out as a metal. One example of a hybrid flow battery is a zinc-bromine battery, where the zinc metal is platted out. In these systems, the power and energy capacities are coupled, and the plating density affects the energy/power capacity ratio.
Redox flow batteries can be utilized in many technologies that require the storage of electrical energy. For example, redox flow batteries can be utilized for storage of night-time electricity (which is inexpensive to produce) to subsequently provide electricity during peak demand when electricity is more expensive to produce or demand is beyond the capability of current production. Such batteries can also be utilized for storage of green energy, i.e., energy generated from renewable sources such as wind, solar, wave, or other non-conventional sources.
Many devices that operate on electricity are adversely affected by the sudden removal of their power supply. Flow redox batteries can be utilized as uninterruptible power supplies in place of more expensive backup generators. Efficient methods of power storage can be used to construct devices having a built-in backup that mitigates the effects of power cuts or sudden power failures. Power storage devices can also reduce the impact of a failure at a generating station.
Other situations where uninterruptible power supplies can be of importance include, but are not limited to, buildings where uninterrupted power is critical, such as hospitals. Such batteries can also be utilized for providing an uninterruptible power supply in developing countries, many of which do not have reliable electrical power sources, resulting in intermittent power availability. Another possible use for redox flow batteries is in electric vehicles. Electric vehicles can be rapidly “recharged” by replacing the electrolyte. The electrolyte can be recharged separately from the vehicle and reused.